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@@ -1,25 +1,65 @@ # cabal-helper [](https://gitlab.com/dxld/cabal-helper/commits/master) -Cabal's little helper provides access to build information gathered by `cabal` -when configuring a project. Specifically we're interested in retrieving enough -information to bring up a compiler session, using the GHC API, which is similar -to running `cabal repl` in a project. - -While simple in principle this is complicated by the fact that the information -Cabal writes to disk is in an unstable format and only really accessible through -the Cabal API itself. - -Since we do not want to bind the user of a development tool which utilises this -library to a specific version of Cabal we compile the code which interfaces with -the Cabal library's API on the user's machine, at runtime, against whichever -version of Cabal was used to write the on disk information for a given project. - -If this version of Cabal is not available on the users machine anymore, which is -fairly likely since cabal-install is usually linked statically, we have support -for compiling the Cabal library also. In this case the library is installed into -a private, isolated, package database in `$XDG_CACHE_HOME/cabal-helper` so as to -not interfere with the user's package database. +The ultimate goal of `cabal-helper` is to give Haskell development tools access +to the same environment which build tools such as +[cabal](https://www.haskell.org/cabal) and [stack](https://www.haskellstack.org) +provide to the compiler. + +## Introduction + +In the Haskell ecosystem the most widely used [build system](#build-system) is +the [`Cabal` library](https://hackage.haskell.org/package/Cabal), not to be +confused with the `cabal` *build tool* which is usually refered to by it's +package-name: `cabal-install` to disambiguate. + +All contemporary meta *build tools* such as `cabal` and `stack` as well as some +custom Haskell [build systems](#build-system) use the `Cabal` library as their +foundation. For example the Glasgow Haskell Compiler's bespoke GNU Make based +build system also utilises the `Cabal` library at its core. + +We capitalize on this fact by using build information `Cabal` writes to disk as +the common denominator between all Haskell *build tools*. This allows us to +easily support a variety of *build tools* without incuring significant +additional complexity. + +## Technical Background + +Essentially all Haskell packages implement +["The Haskell Cabal" (pdf)](https://www.haskell.org/cabal/proposal/pkg-spec.pdf) +packaging specification. The `Cabal` library and `cabal-install` *build tool* +are named after this specification. Yes we really love confusing naming in +Haskell land. + +The specification revolves around this `Setup.hs` script file you might have +seen before. Basically the idea is a Haskell source package consists of, at the +very least, a `Setup.hs` file, which is a Haskell program that provides a well +defined command-line interface for configuring, building and installing +it. Developers can use *build tools*, such as `cabal`, which interface with +`Setup.hs` and provide functionality on top it. + +Even though packages are in principle free to implement the `Setup.hs` interface +however they like in practice everybody just imports the `Cabal` library in +`Setup.hs` which provides a default implementation of this interface. The +`Cabal` library then in turn uses the `<pkg-name>.cabal` file you've likely seen +before to determine precisely what to do. + +Since use of `Cabal` the library is pretty much a given we're just going to be +talking about this case from here on out. + +So the first step of building a package in "The Haskell Cabal" is to call: + + $ ./Setup.hs configure + +The `Cabal` library will then go off and probe a bunch of stuff about the system +it's running on, such as the list of available Haskell packages, compiler type +and version, paths to build tools among other things. Using this information +`Cabal` then writes the concrete configuration of the package into a file called +`setup-config`. Subsequent steps (`./Setup.hs build`...) will then read this +file to avoid probing the system again. + +It is this file that `cabal-helper` is primarily concerned with reading and +presenting in a usable format. ## IRC |